Hollywood Stunts
How do scientific principles "protect" stunt performers as they
perform dangerous feats? What kinds of protective gear do stunt people
use? Why is timing so important in stunts?

Insights

Hardly a movie made today is without some kind of amazing stunt work.
We all are held breathless when a person falls out of a 20-story building
or when a heart-stopping car chase ends in a spectacular crash. While these
may seem to be spontaneous events, every moment of every stunt is carefully
planned and controlled by the scene directors, who have an understanding
of the basic scientific principles at work.

In falls, bodies obtain their speed because of the net acceleration
due to the forces of gravity and air drag.
On the earth, the acceleration rate of a free-falling body is 32 feet per
second per second of fall (9.8 meters per second squared). This means that
for each second the body is falling, its velocity
increases 32 feet per second, up to a limiting velocity of approximately
125 miles per hour.

As the velocity of a falling body increases, so does its momentum.
Momentum is calculated by taking the mass of a body
and multiplying that number by its velocity. When two things crash, it's
the rate of change of momentum that determines the force
(or "wallop") and does the damage.

To reduce the chances of damage or injury, stunt designers use devices
that stretch out the time it takes to stop a body's momentum. These devices
"soften the blow," so to speak, both figuratively and literally.
The longer the period of time used in changing the momentum,
the less force will be released upon impact. Air bags are one of the devices
used because they slow down the impact of the falling body by allowing
the body to displace a large volume of air. The greater the displacement,
the slower the final impact, and the less chance of injury. For very long
falls of several hundred feet, stunt people often use decelerators: long
elastic ropes that serve as a brake to limit the maximum velocity.

In addition to having the proper equipment, stunt people must know about
human anatomy; they must practice a lot; and they must wear the right protective
gear. Even with all this knowledge and training, however, stunt people
can get injured--the scientific principles they use to their advantage
can be just as effective against them.

Connections

1. What are some of the ways that the human body naturally absorbs
energy during exercise?
2. Why is it important that stunt people fall on their backs instead of
face down?
3. What properties should a material have in order to reduce impact or
to prevent a stunt person from being injured by fire?

Vocabulary

acceleration the rate at which the velocity
of a moving object changes over timedrag the retarding force acting on a body moving
through a fluid, parallel and opposite to the direction of motionforce a push or pull that causes a body to accelerate
or change shapegravity the force that makes objects tend to
move toward each otherinsulator a material that blocks the flow of
heat energy from one region to anothermass the amount of matter a body or object contains;
a measure of the inertia of a body or objectvelocity the speed of a body moving in a certain
direction.

Test several different materials to see which are the best insulators
of heat energy. Find the material that will take the longest amount of
time to heat up and the least amount of time to cool down.

Materials

3 cups polystyrene-foam packing peanuts

2 wool socks

3 cups sand

empty 1-lb. coffee can with lid

long Celsius lab thermometer

watch with a second hand OR a stopwatch

paper and pencil

scissors

portable 1000-watt hair dryer

Set up a log sheet as suggested here:

Material

Temp up 10deg.

Temp down 10deg.

Wool
Polystyrene foam
Sand

Stuff the woolen socks inside the can. Put the lid back on the can.

Using scissors, poke a small hole in the lid. Slip the thermometer
through the hole until it is about half-way into the can.

Allow the can to sit for five minutes. Then read the temperature on
the thermometer and record this as your "starting temperature."

Begin heating the outside of the can by moving the hair dryer slowly
around all sides of the can. Using a watch or stopwatch, start timing the
experiment now. See how many seconds it takes for the thermometer to go
up 10 degrees. Record this on your chart as "time up."

When you hit the 10-degree mark, turn off the hair dryer and time how
many seconds it takes for the temperature to go back down to the starting
point. Record this on your chart as "time down." (The temperature
may continue to rise after removing the heat source. Record this on your
chart as the "maximum temperature.")

Repeat the experiment again using the sand and foam peanuts in place
of the wool socks.

Questions

1. Based on your experiments, which materials would make the most effective
liner to protect a stunt person from heat? Why is it necessary for the
material to heat up slowly and cool down quickly?

2. Did any of the materials get significantly hotter after you turned
off the heat? What would cause this to happen?

3. What other materials could you test?

What types of protective gear developed for stunt people have been incorporated
into our daily lives? Which types of protective sports gear use the same
principles as air bags and decelerators? How might you modify this equipment
so that it is even better at reducing the force of impact?

Experiment with how different types of bungee cords and springs could be
used as decelerators. Attach a large weight (e.g., a pail full of sand
with a lid on) to one end of a cord or spring and let it freefall. Try
to quantify your experiments by attaching a spring scale to the upper end
and measuring the maximum force. What properties make for the best results?

Tapes of this episode of Newton's Apple and others are available
from GPN for only $24.95.
Please call 1-800-228-4630.
For information on other Newton's Apple resources for home and school,
please call 1-800-588-NEWTON!

We encourage duplication for educational
non-commercial use!

Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.